Semiconductor memory devices have become more popular for use in various electronic devices. For example, non-volatile semiconductor memory is used in cellular telephones, digital cameras, personal digital assistants, mobile computing devices, non-mobile computing devices and other devices. Electrical Erasable Programmable Read Only Memory (EEPROM) and flash memory are among the most popular non-volatile semiconductor memories.
Both EEPROM and flash memory utilize a floating gate that is positioned above and insulated from a channel region in a semiconductor substrate. The floating gate is positioned between source and drain regions. A control gate is provided over and insulated from the floating gate. The threshold voltage of the transistor is controlled by the amount of charge that is retained on the floating gate. That is, the minimum amount of voltage that must be applied to the control gate before the transistor is turned on to permit conduction between its source and drain is controlled by the level of charge on the floating gate.
When programming an EEPROM or flash memory device, typically a program voltage is applied to the control gate and the bit line is grounded. Electrons from the channel are injected into the floating gate. When electrons accumulate in the floating gate, the floating gate becomes negatively charged and the threshold voltage of the memory cell is raised so that the memory cell is in the programmed state.
Some EEPROM and flash memory devices have a floating gate that is used to store two ranges of charges and, therefore, the memory cell can be programmed/erased between two states, an erased state and a programmed state that correspond to data “1” and data “0.” Such a device is referred to as a binary or two-state device.
A multi-state (or multi-level) flash memory cell is implemented by identifying multiple, distinct allowed threshold voltage ranges. Each distinct threshold voltage range corresponds to a predetermined value for the set of data bits. The specific relationship between the data programmed into the memory cell and the threshold voltage ranges of the memory cell depends upon the data encoding scheme adopted for the memory cells. For example, U.S. Pat. No. 6,222,762 and U.S. Patent Application Publication No. 2004/0255090 both describe various data encoding schemes for multi-state flash memory cells.
A non-volatile storage device may be used as “embedded” memory of a portable computing device (e.g., mobile phone). The non-volatile storage device may be mounted to a circuit board of the portable computing device using reflow soldering (e.g., infrared (IR) reflow soldering). This reflow soldering process is a high temperature process, and can have different effects to the memory reliability depending on the state of memory cells prior to IR reflow soldering. Prior to the reflow soldering, certain data may have been stored in the data storage device using a preloading process. In some situations, one or more blocks within the data storage device may be partially written (e.g., because the total amount of data written to the data storage device is not divisible by the block size of the data storage device). The partially written block(s) may include programmed word lines as well as unprogrammed word lines. Due to effects of heating caused by the reflow process, the unprogrammed word lines may exhibit poor data retention and larger threshold voltage shifting, as compared to other word lines. Thus, certain portions of the data storage device may have lower reliability than other portions of the data storage device.